It is well known in the art that the types and amounts of substances present in engine exhaust is greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures, with excess fuel, tend to produce higher amounts of hydrocarbons and carbon monoxide, whereas lean mixtures, with excess air, tend to produce greater amounts of oxides of nitrogen. It is also known that exhaust gases can be catalytically treated to reduce the amounts of these undesirable components when the air fuel contents of the exhaust gases is maintained within a narrow range of ratios. The catalytic treatment of gases is achieved by a three-way catalytic converter provided that the air-fuel mixture supplied to the catalytic converter is maintained within the narrow range, termed the "converter window". However, this converter window is too narrow to be maintained by an conventional open loop fuel control system, and conversion efficiency drops dramatically for the different undesirable exhaust constituents on either side of the window.
A closed loop fuel control system has been suggested which can maintain the gases supplied to the catalytic converter within the narrow range by a feedback signal from a zirconia sensor exposed to the exhaust gases. However, the design of such a control system must meet a number of requirements. The system must be stable to maintain continual control and not go into oscillation. On the other hand, the system must be quick reacting and characterized by small overshoot, so that the minimum time is spent outside of the converter window.
The zirconia sensor provides an electrical signal representative of the concentration of the oxygen in the exhaust gases. However, this sensor is temperature dependent since its internal impedance is extremely high when the exhaust temperature is low so that the output delivered from the sensor with the engine under cold start remains at low voltage level. Under these circumstances, it is desirable to suspend the closed control operation. It is also desirable to resume feedback control so soon as the temperature of the exhaust gases warrants feedback control.
It is disclosed in Co-pending U.S. patent application Ser. No. 767,133 filed on Feb. 9, 1977 that, in a closed fuel control system, the output from the exhaust gas sensor is compared with a signal representative of the time integral of the sensor output and generates a signal representative of the deviation of the air-fuel ratio in the exhaust system from the time integral or average value of its ratio. Such time integration of the sensor output serves to compensate for the changing characteristics of the sensor with its temperature and aging. However, this time integral signal should be clamped so that its minimum voltage level corresponds to a level that represents the operating temperature of the sensor so that under cold start operation the time integral signal is prevented from going extremely low. Since under these circumstances the output from the exhaust gas sensor rises almost at the same rate as the rate at which the time integral signal rises for a certain interval of time until closed fuel control operation becomes effective, the result of the comparison between the two input variables is indeterminate as long as they take equal values though the sensor's operating temperature is reached. Therefore, the deviation of the air-fuel ratio from its time integral value is uncertain for a certain period of time and closed control operation cannot be quickly commenced.